Pension and Postretirement Benefit Plans - Company's Share of Pension Plans' Available-for-Sale Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2,537.2	$ 2,396.4
Pension
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	200.9	177.6
Pension | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	2.0
Pension | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	113.0	96.3
Pension | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	60.4	51.0
Pension | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	48.0	38.9
Pension | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	4.6	6.4
Pension | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	59.6	59.0
Pension | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	26.3	22.3
Pension | Quoted prices in active markets for identical assets (Level 1)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	85.9	81.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	59.6	59.0
Pension | Quoted prices in active markets for identical assets (Level 1) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	26.3	22.3
Pension | Quoted prices in active markets for identical assets (Level 1) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0
Pension | Significant other observable inputs (Level 2)
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	115.0	96.3
Pension | Significant other observable inputs (Level 2) | Fixed maturities:
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	113.0	96.3
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. treasury securities and obligations of U.S. government agencies
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	60.4	51.0
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | Corporate securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	48.0	38.9
Pension | Significant other observable inputs (Level 2) | Fixed maturities: | U.S. government agencies mortgage-backed securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	4.6	6.4
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Large-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	0.0
Pension | Significant other observable inputs (Level 2) | Equity Securities [Member] | Small-cap securities
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	0.0	$ 0.0
Pension | Significant other observable inputs (Level 2) | Short-term Investments [Member]
Schedule Of Available For Sale Securities Fair Value Hierarchy [Line Items]
Available-for-sale Securities	$ 2.0